A large class of naturally occurring compounds like antibiotics, toxins, hormones, ion transport regulators and other biologically active compounds are cyclic peptides. Synthetic cyclic peptides of defined backbone conformation are potentially useful as analogs for determining biologically active conformations of naturally occurring peptides. The conformational restrictions imposed by cyclisation make cyclic peptides excellent model compounds for the study of the protein conformation. Many cyclic peptides also take part in the transport of ions across biological membranes. X-ray crystallography is a fine tool to determine the conformational features of cyclic peptides. The conformation of a molecule is intimately related to its biological activity and the study of the conformation of cyclic peptides crystallized under different conditions and comparison of the results with those obtained from other methods such as spectroscopic techniques would be very valuable. In this project using X-ray diffraction technique the examination of the structure and conformation of a few synthetic cyclic peptides which are useful as analogs for biologically active molecules and the study of metal ion complexes of cyclic peptides which are useful in understanding the ion transport mechanism across the biological membrane are planned.